Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

On Fri, Sep 19, 2014 at 3:19 PM, Filipe Brandenburger
[off-list ref] wrote:

Hi,

On Fri, Sep 19, 2014 at 3:09 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Fri, Sep 19, 2014 at 3:02 PM, Filipe Brandenburger [off-list ref] wrote:

quoted

In case that's useful, I was looking at swapping the vvar page by
changing the vm_special_mapping to change the pages array between the
actual vvar page and the zero page and using zap_page_range to force
the next access to go through a page fault that would remap it.

That will do it globally, since the vm_special_mapping is global.  I
was just thinking of using remap_pfn_range.

Sorry if I wasn't clear... That's the exact point of my patch, to make
vm_special_mapping local to the task.

I started with an approach of keeping a struct vm_special_mapping + a
struct page * array per mm_struct.

I was also looking at keeping two static vm_special_mapping structs,
one with the actual vvar page and the other with the zero page and
then swapping vma->vm_private_data to point to the appropriate one.

This sounds like it may be more complicated than necessary.  Is there
any reason that just doing remap_pfn_range on the vvar page isn't
enough?

Changing out the text is a whole can of worms involving self-modifying
code, although it may be completely safe if done through the page
tables.  But I don't think you can't use remap_pfn_range for that.

quoted

Give me another day or two to straighten out the vma stuff, although
it shouldn't impact your patches too much.  The main effect will be
that you'll be able to rely on mm->context.vdso (renamed to
vvar_vma_start, most likey) being correct.  Currently, you should
*not* rely on it, especially if CRIU is involved.

Cool! As I said, let me know if you'd like to see some of my
incomplete patches or if you'd like me to keep working on them to show
you something more complete.

Maybe better: what exactly are you trying to do?

--Andy